Immunology Structure And Functions Of Immune System

Structure And Functions Of Immune System

Question 1. Write a short note on antigen processing cells and major histocompatibility complex (MHC) restriction.
Answer:

Antigen-processing cells are also known as antigen-presenting cells.

An antigen-presenting cell (APC) is a cell that displays foreign antigens complexed with major histocompatibility complexes (MHC) on their surfaces. T-cells may recognize these complexes by using their T-cell receptors (TCRs). These cells process anti- gens and present them to T-cells.

Types An antigen-presenting cell

APCs fall into two categories:

Professional or non-professional. T-cells cannot recognize, and therefore cannot respond to, ‘free’ antigen. T-cells can only ‘see’ an antigen that has been processed and presented by cells via carrier molecules like MHC and CD1 molecules.

Most cells in the body can present antigens to CD8+ T cells via MHC class I molecules and thus, act as “APCs”. These cells, in general, express MHC class II as well as MHC class I molecules and can stimulate CD4+ (“helper”) cells as well as CD8+ (“cytotoxic”) T cells, respectively.

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Professional APCs

Professional APCs are very efficient at internalizing antigens,either by phagocytosis or by receptor-mediated endocytosis, and then displaying a fragment of the antigen bound to a class II MHC molecule on their membrane.

The T cell recognizes and interacts with the antigen-class II MHC molecule complex on the membrane of the antigen-presenting cell. An additional costimulatory signal is then produced by the antigen-presenting cell leading to the activation of the T cell. The expression of costimulatory molecules is a defining feature of professional APCs.

There are three main types of professional antigen-presenting cell:

• Dendritic cells, have the broadest range of antigen presentation, and are probably the most important APC.
• Activated dendritic cells are especially potent Th cell activators because, as part of their composition, they express co-stimulatory molecules such as B7.
• Macrophages, which are also CD4+ cells and are therefore also susceptible to infection by HIV.
• Certain B-cells which express (as B cell receptor) and secrete a specific antibody, can internalize the antigen, which bind to its B cell receptor and present it incorporated to MHC II molecule, but are inefficient APC for most other antigens.
• Certain activated epithelial cells.

Non-professional APC

A non-professional APC does not constitutive express the Major histocompatibility complex class II (MHC class II) proteins required for interaction with naive T cells; these are expressed only upon stimulation of the non-professional APC by certain cytokines such as IFN-γ. Non-professional APCs include:

• Fibroblasts (skin)
• Thy mic epithelial cells
• Thyroid epithelial cells
• Glial cells (brain)
• Pancreatic beta cells
• Vascular endothelial cells.

Major Histocompatibility Complex Restriction

• T cells accept the processed antigens on the macrophages only when both the cells, i.e. T cells and macrophage, possess surface determinants coded by the same MHC antigen.
• This is known as MHC restriction.
• Both class I and class II antigens operate in this phenomenon.
• Cytotoxic T lymphocytes from immunized mice are able to kill and lyse virus-infected target cells only when the T cells and target cells are of the same MHC type so that the T cells can recognize Class I MHC antigens on the target cells.
• T helper cells can accept antigens presented by macrophages only when the macrophages bear the same Class II MHC molecules on the surface.
• For T cells participating in delayed-type hypersensitivity, the antigen has to be presented along with Class II MHC determinants.

Question 2. Write a short note on T-lymphocytes—its types and specific functions of them.
Answer:

T cells originate from the precursor cells of bone marrow, migrate to thymus and mature there.

Types and Functions of T-Lymphocytes

1. Helper/Inducer (Th) cells:

Also known as CD4 cells. They are with a CD4 surface marker and MHC class II restriction. They stimulate and promote the growth of T cells and macrophages.

They are of two types:

• TH1 cells: They produce cytokines, interferon-gamma and interleukin-2 which activate macrophage and T cells, promoting cell-mediated immunity, destruction of target cells and killing of intracellular pathogens.
• TH2 cells: They are principal helper cells. They produce cytokines, interleukin 4, 5 and 6 which stimulate B cells to form antibodies.

2. Suppressor T-Cells (Ts):

Also known as CD8 cells. They possess CD8 surface markers. They block antibody production by acting on Th cells or by acting directly on B cells by secreting suppressor substances.

3. T – regulator cells (Tr):

These are the regulator cells that regulate the activity of Th and Ts cells.

4. Cytotoxic T-Cells (Tc):

They possess CD8 surface markers and MHC class I restriction, which can kill and lyse target cells carrying new or foreign antigens. They cause destruction of target cells by releasing lymphotoxins and perforin.

5. Delayed hypersensitivity T cells (TD cells):

They possess CD4 markers. These cells secrete diffrent lymphokines which are responsible for inflammatory response of de- layed hypersensitivity and growth factors which are believed to regulate lymphocyte activity.

Question 3. Write a short note on T-lymphocyte and B-lymphocyte interaction in the immune system of host.
Answer:

T-lymphocytes, i.e. T helper lymphocytes (Th2) interact with B lymphocytes to develop them into plasma cells which produce immunoglobulins in the immune system of the host. This interaction is mediated by lymphocytes secreted by T helper cells.

• Antigenically stimulated B lymphocytes undergo blast transformation and become plasmablasts,
  intermediate transitional cells and then plasma cells.
• A plasma cell can synthesize antibody which provides protection from antigenic responses in the immune system of the host.

Question 4. Write a short note on cells and their mechanism involved in Phagocytosis.
Answer:

Cells involved in phagocytosis Cells of phagocytosis are macrophages and microphages.

Phagocytosis Macrophages:

• Macrophages are large round or oval cells with kidney-shaped nuclei and abundant cytoplasm.
• Blood macrophages, i.e. monocytes are 12 to 15 µ in size, while tissue macrophages, i.e. histiocytes are 15 to 20 µ in size. All these cells are widely distributed in the body.
• Macrophages are produced from the stem cells of the bone marrow where they proliferate and are supplied as monocytes in the bloodstream as they get mature.
• Blood monocytes after a few days migrate to tissues and become macrophages. These tissue macrophages are present in lung and liver.
• Blood monocytes have a half-life of 3 days, while tissue macrophages survive for months because they proliferate locally.

Phagocytosis Microphages

• Macrophages are the small non-dividing polymorphonuclear leukocytes or granulocytes which are present in the blood.
• Polymorphonuclear macrophages of blood are neutrophils, eosinophils, and basophils.
• Mainly, neutrophils and to a lesser extent eosinophils are phagocytic in nature. These cells consist of granules and bactericidal substances.
• Microphages originate from stem cells in bone marrow, undergo maturation, and are released in circulation.
• Microphages have a half-life of 2 days in circulation and are short-lived. As they penetrate in tissue, they have a life of a few hours.

Mechanism of Cells Involved in Phagocytosis

• Macrophage: Macrophage envelopes the foreign particle by small pseudopodia. Membranes of pseudopod get fused and form phagosome.
  Lysosome having hydrolytic enzymes undergoes fusion with the membrane of the phagosome and become phagolysosome. Most of the bacteria are killed intracellularly by these lysosomal enzymes.
• Neutrophils: It has non-specific phagocytic property.
• It plays an important role in phagocytosis in acute inflammation by locating a foreign particle.
• Eosinophils: They are less phagocytic as compared to neutrophils, but their role in phagocytosis is still unclear.
• Basophils: The cytoplasm of basophils has basophilic
  granules which have heparin, histamine, serotonin, and various other hydrolytic enzymes. Mast cell degranulation releases these agents, which causes anaphylaxis and atopic allergy.